Understanding Tsunamis: Causes and Stages

Tsunamis: Definition

• A tsunami is a series of powerful ocean waves that surge onto land, often reaching up to 100 feet (30 meters) in height.
• "Tsu" means harbor referring to coastal areas, and "nami" means wave.

Causes of Tsunamis

• Earthquakes are the most common cause; specifically, the shifting of tectonic plates at subduction zones.
• Landslides, whether underwater or coastal, can displace significant amounts of water.
• Volcanic eruptions or the collapse of volcanic islands are also causes.

Tsunami Stages

• Initiation: A sudden underwater disturbance displaces a large volume of water.
• Propagation: The displaced water forms waves that travel outward at high speeds.
  • In deep water, these waves are barely noticeable but gain energy.
• Amplification: As the tsunami enters shallow coastal waters, its speed decreases, and its height increases dramatically.
• Inundation: The waves crash onto the shore, flooding coastal areas, with the "run-up" being the maximum vertical height the wave reaches inland. Waves can sweep away buildings, trees, and people.

Effects of Tsunamis

• Causes severe flooding in coastal regions.
• Results in loss of life and property destruction.
• Leads to contamination of water supplies and potential disease outbreaks.
• Causes economic damage by destroying businesses and farmlands.
• The "run-up" signifies the maximum vertical height a tsunami achieves inland above sea level, indicating the severity of flooding and damage.

Formation of Tsunamis

• Tsunamis form when a large volume of water is displaced, often due to tectonic activity, including underwater earthquakes (most common), volcanic eruptions, and landslides.
• Once water is displaced, it moves outward in all directions, creating tsunami waves.

Subduction Zones

• Subduction zone: an area where one tectonic plate slides beneath another into the mantle, typically at convergent plate boundaries.
• As one plate slides and gets stuck, pressure builds until the plates suddenly slip, causing an earthquake that lifts or lowers the ocean floor, resulting in tsunami waves spreading outward.
• The 2004 Indian Ocean tsunami was caused by an earthquake along the Sunda Megathrust subduction zone.

Seismic Waves & Energy Transfer

• Earthquakes release energy in seismic waves.
• Energy is transferred vertically, lifting or lowering the ocean floor, displacing water, and forming a tsunami.
• Tsunami waves can travel up to 800 km/h (500 mph) in deep water, slowing down but increasing in height as they reach shallow waters.

Tsunami Waves vs Regular Ocean Waves

Tsunami-Prone Areas

• Tsunamis mostly occur near subduction zones and active tectonic plates.
• About 80% of all tsunamis occur in the Pacific Ocean, especially in the Pacific Ring of Fire.
• The Pacific Ring of Fire is a zone around the Pacific Ocean with frequent earthquakes and volcanic eruptions, containing 75% of the world’s active volcanoes and 90% of all earthquakes.
• Subduction zones in this area cause underwater earthquakes.
• Frequent earthquakes are due to countries on active fault lines, such as Japan, the Philippines, and Indonesia.
• Underwater volcanic activity displaces water.

Examples of Tsunami-Prone Countries in the Pacific Ring of Fire:

• Philippines: prone to tsunamis due to the Philippine Trench and frequent earthquakes.
• Japan: experienced the 2011 Tōhoku tsunami.
• Indonesia: affected by events, including the 2004 Indian Ocean tsunami.

Indian Ocean Tsunamis

• The 2004 Indian Ocean tsunami occurred on December 26, 2004.A magnitude 9.1 earthquake struck off the coast of Sumatra, Indonesia, causing the ocean floor to rise, displacing water and creating tsunami waves that hit 14 countries.
• Over 230,000 people died, entire villages were wiped out, and millions were left homeless.
• The tsunami reached speeds of 800 km/h and waves as high as 30 meters (100 feet).

Historical Tsunamis in the Philippines

• The Philippines is susceptible to tsunamis.
• The 1976 Moro Gulf Tsunami occurred on August 17, 1976, in Moro Gulf, Mindanao caused by a magnitude 8.1 earthquake.
• Waves reached up to 9 meters (30 feet), affecting Zamboanga, Pagadian, Cotabato, and Jolo.
• More than 5,000 people died.
• Factors that contributed to fatalities: absence of a tsunami warning system, heavily populated coastal communities, and strong waves and flooding.
• After the tsunami, the Philippines developed tsunami early warning systems, and now, coastal communities are more aware of tsunami signs and safety measures.

Key Facts

• Underwater earthquakes trigger tsunamis when tectonic plates shift at subduction zones.
• Subduction zones generate tsunamis as built-up pressure releases, causing the ocean floor to shift.
• Seismic waves from underwater earthquakes displace water, creating a tsunami.
• Tsunamis increase in height as they approach the shore due to the compression of wave energy in shallow water.
• Tsunamis have longer wavelengths and travel faster than regular ocean waves.

Essential Factors

• Earthquakes must occur underwater or near a large body of water.
• Subduction zone earthquakes are more likely to generate tsunamis.
• Earthquakes must be strong (typically magnitude 7.0 or higher) to displace water.
• Shallower earthquakes (around 10-50 km deep) are more likely to cause tsunamis.
• Underwater landslides or volcanic eruptions can also trigger tsunamis.